Club 80-90 Forums

Please can an electrical guru help me?
I have recently acquired a Zig unit. I have put 240acv on but I'm only getting 9dcv out.
I measured 14acv output from the transformer terminals and 9dcv on the TRU output terminals.
I believe the transformer is the culprit as the TRU output is correct for its input (14 * 0.707).
Can a transformer give up/decay?

I used this schematic from the archives:
http://archive.club8090.co.uk/viewtopic ... 9fec1d364f" onclick="window.open(this.href);return false;

Thanks BM

Any suggestions please?

I hope somebody who knows will see this thread and give you a proper answer. I can only offer another question I'm afraid. You measured 14VAC at the output of the transformer. Does your meter (and I guess the question is true for all AC voltage meters) read the peak or the rms? If it's the peak, then your comment about the rectifier seems right, but if it's already reporting the rms then the output of the TRU should be higher shouldn't it? If 14VAC is the rms then shouldn't the rectifier output 14VDC?

I'm way out of my depth here. I think Ghost is the right person to chip in - hopefully he'll see this thread.

I have this ZIG unit in my van, so if you don't get the answer you need I may be able to pull the cover off and check your measurements against mine.

If 14VAC is the rms then shouldn't the rectifier output 14VDC?

Would the output from the rectifier be minus the voltage drop across the diodes? My CF8 puts out about 13v with a leisure battery connected I think?

Thanks Gents. Your comment about RMS got me thinking. I found this link:
http://electronicsclub.info/powersupplies.htm" onclick="window.open(this.href);return false;

Maybe 14acv from the transformer is right. From the link:

The rectifier unit should consume 1.4v and full wave rectification should give me 12.6v (14-1.4 = 12.6). This would be the RMS voltage I need. If I'm only getting half rectification then this will reduce the output. (12.6 * 0.707) = 8.9v.
Or, since peak is 1.4 times RMS (12.6 * 1.4) = 17.64 peak but since only half the cycles are rectified the voltage is halved = 8.82v. Which is spookily the dc voltage I'm getting!

Am I clutching at straws?

So you think one of the diodes is broken so you're only getting half rectification? Seems feasible I guess.

Ghost! I hope you ears are burning.

Yep - that's my diagnosis, I just can't decide which of the 2 theories applies (if any)!

I don't think Ghost is coming out to play....

Ok, I tested the rectifier and found that each diode was connected and taking around 0.7V. This confused me as I'm still getting around 9V. This is when I had a brain wave - I checked the voltage at the dc terminals of the rectifier - 12.3V! On my previous checks i had been measuring everything using the chassis as my reference point (that's where I put the negative test probe) hence I was getting 9V. The problem seems to be the SCR or the Zener Diode - that's where the voltage is dropping from 12 to 9V. Why?

I am here

Trouble with this type of thing is it's so frustrating trying to do it remotely. If I was there I could identify the fault in less than 5 minutes. Also Bubble Meister seems to have a logical grasp of what he/she is doing. Plus although electronics is my main subject, I don't like and have rarely worked on CF8's. I find them over complicated for what they do.

However, my advice = don't get bogged down in all the technicalities and the maths. Well not for the purposes of fixing it anyway. You meter is all you need. Start with the simple stuff, ie checking all connections and earths. I wouldn't go around checking voltages though. It can muddy the water. Look for a fault and once sorted the voltages will then be correct.

Next you can test the semiconductors with your meter, but this is best done with them out of the circuit to avoid false readings. This involves removing and replacing them (soldering). So, new parts (semiconductors) off ebay would likely come to less than ~£6. To save all the learning how to test SCR's and Zenners etc, I would by new parts and fit them. It 90% will then work (unless you missed a bad connection )

p.s. Bridge rectifier (or does it use 4 x diodes?), SCR and Zenner are all "generic" and non specific, so as long as you order the correct Voltage and current ratings you can't go wrong. (assuming you know about fitting them the correct way round? Make notes or take pictures )
After having said all that, if you would like to ask me any specific questions about anything, please feel free to do so

Thanks for your reply Ghost. I've tried all the logical stuff with all the components still soldered in. I'm hesitant to disconnect soldered components as I'm not handy with a soldering iron (I only started using one in the last couple of weeks and the results are still hit and miss though more miss!).
Can you explain what the purpose of the Zener, the resistor and the SCR are in the circuit? If I can understand that then maybe I can start to eliminate components but also it adds to my knowledge bank!

Bubble Meister wrote: Can you explain what the purpose of the Zener, the resistor and the SCR are in the circuit? If I can understand that then maybe I can start to eliminate components but also it adds to my knowledge bank!

I was dreading you asking that

I have 30+ years of (sometimes) professional experience repairing electronic circuits (Hi-Fi's, CB radio's etc etc) and even after studying that diagram I am confused. It is obviously some sort of voltage regulation circuit to provide a charging voltage (likely around 13.8V), or maybe not, maybe it's a non regulated "charger" working on a current limit from the transformer, in which case the voltage from earth to the output of the bridge, with no load on it, could be quite high. From what I can figure out, the SCR is acting as an electronic switch to earth, but why and how the 5.1V zenner and it's current limiting resistor is controlling the gate of the SCR is a mystery to me at present (which is another reason I suggested testing them out of circuit, rather than figure out what voltages "should" be present ) I have sent a pm to AngeloEvs who is also hot on this subject to see if he can figure out what they are doing and post on here.

Bubble Meister wrote:Any suggestions please?

I'd try connecting various small loads & see what the voltage is then. Also there isn't any capacitance in the ZIG unit so you won't get proper DC readings as the voltage won't be smoothed at all. (Usually the leisure battery would act as the capacitor)

Thanks Windy. I can appreciate that the leisure battery would act as the capacitor but there is 12.3v across the two dc terminals of the rectifier yet only 9V from the rectifier positive dc terminal to the earth (0v).

I'm pretty sure I checked if was any ac output from the rectifier but got 0acv.

At the moment you are losing 3.3v across the SCR circuit, but its "seeing" no load. As previously stated try putting a load on the output & see what happens to the voltage then.